1. Field of the Invention
The present invention relates to a method for detecting leading and trailing ends of a video tape in a magnetic recording/reproducing apparatus such as a video cassette tape recorder.
2. Description of the Prior Art
Where a video tape runs in a magnetic recording/reproducing apparatus, it should stop its running at its leading or trailing end. If not, high tension is then applied to the video tape due to the drive force of a motor adapted to run the video tape or inertia force generated by the rotation of a hub on which the video tape is wound. In this case, the video tape may be damaged. For example, the tape may be cut or elongated.
Such damage of the video tape becomes more severe when the speed of the video tape is increased.
In order to solve the above-mentioned problem, conventional magnetic recording/reproducing devices are provided with an end detecting means adapted to detect the leading and trailing ends of the video tape of a tape cassette which is loaded in the magnetic recording/reproducing device to record video and audio signals thereon while reproducing those signals.
When the end detecting means detects the leading or trailing end of the video tape, the magnetic recording/reproducing device stops the running of the video tape in response to the detection of the end detecting means so as to prevent the video tape from being damaged.
Such a conventional configuration will now be described in detail in conjunction with FIGS. 1 and 2.
FIG. 1 is a perspective view schematically illustrating the deck unit of a conventional magnetic recording/reproducing apparatus which includes an end detecting means.
In FIG. 1, the deck unit is denoted by the reference numeral 1. The reference numeral 2 denotes a tape cassette which is loaded in the deck unit 1 to record video and audio signals thereon while those signals are being reproduced.
The reference numerals 3 and 4 denote a supply reel and a take-up reel respectively fitted in the supply and take-up hubs of the tape cassette 2 when the tape cassette 2 is loaded in the deck unit 1. When the deck unit 1 operates to run the video tape, the supply and take-up reels 3 and 4 rotate, thereby causing the supply and take-up hubs of the tape cassette 2 to rotate.
A vertical support member 5 having a certain height is disposed in the deck unit 1 to the rear of the supply and take-up reels 3 and 4. A light emitting element 6 is mounted on the upper end of the support member 5.
A pair of light receiving elements 7 and 8 are disposed at both side portions of the deck unit 1, respectively. The light receiving element 7 serves to detect the trailing end of the video tape whereas the light receiving element 8 serves to detect the leading end of the video tape.
FIG. 2 is a perspective view illustrating a tape cassette which is of a general video home system (VHS) standard.
In FIG. 2, the tape cassette is denoted by the reference numeral 2. The reference numerals 21 and 22 denote supply and take-up hubs included in the tape cassette 2, respectively. The reference numeral 23 is a video tape wound on the supply and take-up hubs 21 and 22.
The video tape 23 is coated with a magnetic material to record video and audio signals thereon while reproducing those signals. The video tape 23 comprises an opaque magnetic tape 23a and a pair of transparent lead tapes 23b.
The lead tapes 23b are provided at the supply and take-up hubs 21 and 22, respectively. That is, each lead tape 23b is attached at one end thereof to an associated end of the magnetic tape 23a. The lead tapes 23b are also attached at the other ends thereof to the supply and take-up hubs 21 and 22, respectively.
When the tape cassette 2 is loaded in the deck unit 1 of the magnetic recording/reproducing apparatus, the support member 5 and light emitting element 6 are inserted into an insertion groove formed on the lower surface of the tape cassette 2. The supply and take-up reels 3 and 4 are also fitted in the supply and take-up hubs 21 and 22 of the tape cassette 2, respectively.
When the magnetic recording/reproducing apparatus operates in a play-back, fast play-back, reverse play-back, fast forward or rewind mode under the above-mentioned condition, the light emitting element 6 turns on, thereby emitting light beams. The light beams from the light emitting element 6 are directed to the light receiving elements 7 and 8, respectively.
As the supply and take-up reels 3 and 4 rotate, the supply and take-up hubs 21 and 22 rotate, thereby causing the video tape 23 to run.
Where the tape running is carried out at the intermediate portion of the video tape 23, the opaque magnetic tape 23a runs across light beam paths respectively defined between the light emitting element 6 and the associated light receiving elements 7 and 8. As a result, the light beams emitted from the light emitting element 6 are shielded by the magnetic tape 23a, so that they can not be incident on the light receiving elements 7 and 8. In this case, no detect signal is outputted from the light receiving elements 7 and 8.
In this state, a control unit (not shown), which is adapted to control the magnetic recording/reproducing apparatus, discriminates that the leading or trailing end of the video tape 23 has not been detected yet. Accordingly, the control unit controls the magnetic recording/reproducing apparatus to continuously run the video tape 23 in the current operation mode.
Where the tape running is carried out at the trailing end portion of the video tape 23, the associated transparent lead tape 23b runs across the light beam path defined between the light emitting element 6 and light receiving element 7. As a result, the light beam emitted from the light emitting element 6 passes through the transparent lead tape 23b and then enters the light receiving element 7. Accordingly, the light receiving element 7 outputs a trailing end detect signal.
On the other hand, when the tape running is carried out at the leading end portion of the video tape 23, the associated transparent lead tape 23b runs across the light beam path defined between the light emitting element 6 and light receiving element 8. As a result, the light beam emitted from the light emitting element 6 passes through the transparent lead tape 23b and then enters the light receiving element 8. Accordingly, the light receiving element 7 outputs a leading end detect signal.
Based on the trailing end detect signal from the light receiving element 7 or the leading end detect signal from the light receiving element 8, the control unit discriminates that the trailing or leading end of the video tape 23 has been detected.
Accordingly, the control unit controls the magnetic recording/reproducing apparatus to complete the current operation mode while stopping the running of the video tape 23 so as to prevent the video tape from being damaged.
As is apparent from the above description, the conventional configuration includes separate light emitting and receiving elements to detect the leading and trailing ends of the video tape.
The light emitting element is fixedly mounted to the support member upwardly protruded from the intermediate portion of the deck unit whereas the light receiving elements are fixedly mounted to both side portions of the deck unit by means of printed circuit boards, respectively.
Due to such a configuration for mounting the light emitting and receiving elements, the assembling process for the deck unit is complex. This results in an increase in the manufacturing costs and a degradation in productivity.
The conventional magnetic recording/reproducing apparatus should also have an additional light shielding means to prevent external light from entering the light receiving elements. In this case, it is difficult to obtain a compact construction for the deck unit since the deck unit should have a space for such a light shielding means.
As mentioned above, the light emitting and receiving elements are installed in the deck unit in such a manner that light beams emitted from the light emitting element are incident on the light receiving elements via the lead tape portions of the video tape, respectively.
For this reason, the tape cassette is provided with light beam paths for allowing the light beams emitted from the light emitting element to enter the light receiving elements via the lead tape portions of the vide tape, respectively.
Generally, the video tape wound on the hubs of the tape cassette may have various lengths in accordance with the type of the tape cassette based on the total running time in a normal running mode. For example, tape cassettes are classified into those for 10 minutes, 20 minutes, 30 minutes, 45 minutes, 60 minutes, 80 minutes, 100 minutes, 160 minutes, 180 minutes and 200 minutes.
For such tape cassettes, two standard hub sizes are used. That is, a hub having a large diameter of 31.02 mm is used for tape cassettes for 10 minutes, 20 minutes, 30 minutes, 45 minutes and 60 minutes. A smaller-size hub having a diameter of 13.06 mm is used for tape cassettes for 80 minutes, 100 minutes, 160 minutes, 180 minutes and 200 minutes.
There is a rule for the maximum diameter of the wound video tape in the case in which the smaller-size hub is used. In accordance with the rule, the maximum diameter of the wound video tape should be equal to or smaller than the diameter of the larger-size hub, namely, 31.02 mm.
For this reason, tape cassettes for a longer running time use a thinner video tape.
For instance, tape cassettes for 120 minutes use a video tape having a thickness of about 20 .mu.m, tape cassettes for 160 minutes use a video tape having a thickness of about 16 .mu.m, tape cassettes for 180 minutes use a video tape having a thickness of about 15 .mu.m, and tape cassettes for 210 minutes use a video tape having a thickness of about 14 .mu.m, so that the maximum diameter of the video tape in a wound state is equal to or smaller than 31.02 mm.
Meanwhile, magnetic recording/reproducing devices are provided with a capstan motor. In an operation mode for recording or reproducing video and audio signals, the capstan motor drives to run the video tape.
In a standard play (SP) mode, the capstan motor serves to run the video tape at a speed of 33.35 mm/sec.
A signal detecting means such as a Hall sensor is disposed at an appropriate position in the vicinity of the capstan motor. When the capstan motor drives, the signal detecting means detects the rotation of the capstan motor, thereby generating a capstan frequency generator (CFG) signal.
The CFG signal has a frequency of 1,080 Hz when the video tape runs in the SP mode. In an SLP mode, the CFG signal has a frequency of 360 Hz.
A number of research efforts are being made to discriminate the hub size of tape cassettes using the above-mentioned tape cassette standards and CFG signals, thereby discriminating the leading and trailing ends of the video tape.
In accordance with techniques in which the hub size is discriminated without using any detecting means, one revolution of each of the supply and take-up reels is detected during the running of the video tape. In this case, the number of CFG signals generated for one revolution of each of the supply and take-up reels is counted. Based on the counted values, the discrimination value for the hub size is calculated using the following equation (1): EQU Pc.sup.2 =Ps.sup.2 +Pt.sup.2 [Equation 1]
where, "Pc.sup.2 " represents the discrimination value for the hub size; "Ps" represents the number of CFG signals generated for one revolution of the supply reel; and "Pt" represents the number of CFG signals generated for one revolution of the take-up reel.
The discrimination value for the hub size, Pc.sup.2, in the equation (1) is constant irrespective of the running condition of the video tape.
Now, the reason why the discrimination value for the hub size, Pc.sup.2, is constant will be described.
First, it is assumed that, in the case of a tape cassette shown in FIG. 3, "Rh" represents the radius of each of the supply and take-up hubs 21 and 22, "Rs" represents the radius of the video tape 23 wound on the supply hub 21, and "Rt" represents the radius of the video tape 23 wound on the take-up hub 22.
In this case, the total area of the video tape wound on both the supply and take-up reels 21 and 22 can be calculated as expressed by the following equation (2): EQU S="Rs.sup.2 +"Rt.sup.2 -2"Rh.sup.2 [Equation 2]
where, "S" represents the total area of the video tape wound on the supply ("Rs) and take-up reels 21 and 22; and "2"Rh.sup.2 " represents the sum of the area of the supply hub 21 and the area of the take-up hub 22.
The total length of the video tape 23 can be calculated by dividing the total area S of the video tape 23 derived from the equation (2) by the thickness of the video tape 23, as expressed by the following equation (3) : EQU L=("Rs.sup.2 +"Rt.sup.2 -2"Rh.sup.2)/Td EQU ="(Rs.sup.2 +Rt.sup.2 -2Rh.sup.2)/Td [Equation 3]
where, "L" represents the total length of the video tape 23; and "Td" represents the thickness of the video tape 23.
Among distance, speed and time, a relationship expressed by "speed=distance/time" is generally established. Based on such a relationship, the total running time of the video tape 23 at a constant running speed can be calculated as expressed by the following equation (4) : EQU T=L/Vo EQU ={"/(Td.times.Vo)}.times.(Rs.sup.2 +Rt.sup.2 -2Rh.sup.2) [Equation 4]
where,
"T" represents the total running time of the video tape 23; and PA1 "Vo" represents the running speed of the video tape 23. PA1 2 "Cs" represents the circumference of the video tape 23 wound on the supply hub 21; PA1 "Ct" represents the circumference of the video tape 23 wound on the take-up hub 21; PA1 "Ps" represents the number of CFG signals generated for one revolution of the supply reel; PA1 "Pt" represents the number of CFG signals generated for one revolution of the take-up reel; and PA1 "F" represents the frequency of the CFG signal.
The circumference of the video tape 23 wound on each of the supply and take-up hubs 21 and 22 at a certain running time point can be calculated as expressed by the following equation (5): EQU Cs=2"Rs=Vo.times.Ts=Vo.times.Ps/F EQU Ct=2"Rt=Vo.times.Tt=Vo.times.Pt/F [Equation 5]
where,
From the equation (5), the radius Rs of the video tape 23 wound on the supply hub 21 and the radius Rt of the video tape 23 wound on the take-up hub 22 can be derived as expressed by the following equation (6): EQU Rs=(Vo.times.Ps)/(2"F) EQU Rt=(Vo.times.Pt)/(2"F) [Equation 6]
When the equation (6) is substituted for the equation (4), the total running time of the video tape 23 can be calculated as expressed by the following equation (7): EQU T={"/(Td.times.Vo)}.times.{(Vo/2"F).sup.2 .times.(Ps.sup.2 +Pt.sup.2)-2Rh.sup.2 } EQU ={Vo/(Td.times.4"F.sup.2)}.times.(Ps.sup.2 +Pt.sup.2)-{2Rh.sup.2 /(Td.times.Vo)} [Equation 7]
The values "Td.times.4"F.sup.2 " and "2Rh.sup.2 /(Td.times.Vo)" in the equation (7) are constant irrespective of the running condition of the video tape.
Since the value "T" in the equation (7), namely, the total running time of the video tape 23, is also constant, the value "Pc.sup.2 " in the equation (1) (Pc.sup.2 =Ps.sup.2 +Pt.sup.2) is constant.
In accordance with the above-mentioned principle, the discrimination value for the hub size, Pc.sup.2, is calculated, as expressed in the equation (1), using the number of CFG signals, Ps, generated for one revolution of the supply reel and the number of CFG signals, Pt, generated for one revolution of the take-up reel.
Based on the calculated hub size discrimination value Pc.sup.2, a retrieval of data previously stored is then executed to discriminate the hub size.
For example, the discrimination of the hub size is achieved using the graph of FIG. 4 which shows the relation between "Ps2" and "Pt.sup.2 ". When the calculated hub size discrimination value is larger than a discrimination reference value ("Pc.sup.2 " in FIG. 4), the hub size is discriminated as a large hub size. On the other hand, when the calculated hub size discrimination value is smaller than a discrimination reference value, the hub size is discriminated as a small hub size.
FIG. 5 illustrates kinds and hub sizes of tape cassettes depending on different hub size discrimination values Pc.sup.2.
In FIG. 5, the hub size discrimination values Pc.sup.2 are expressed in hexadecimal form. "T10", "T20", "T30", "T45", "T60", "T80", "T100", "T120", "T160", "T180", and "T200" represent tape cassettes for 10 minutes, 20 minutes, 30 minutes, 45 minutes, 60 minutes, 80 minutes, 100 minutes, 160 minutes, 180 minutes and 200 minutes, respectively.
Based on the discriminated hub size, the rotating speed of each reel in the current operation mode is determined. Using the determined rotating speeds of the reels, a detection for the leading and trailing ends of the video tape is carried out.
The rotating speed of each reel varies depending on the amount of the video tape wound on the associated hub. Based on this fact, the conventional techniques determine whether or not the rotating speed of each reel is equal to or less than a predetermined reference value. When the rotating speed of each reel is equal to or less than a predetermined reference value, it is discriminated that the leading or trailing end of the video tape is detected, thereby stopping the running of the video tape.
However, such conventional methods involve a lot of errors in detecting the leading and trailing ends of the video tape. The conventional methods may erroneously determine the running position of the video tape. For example, the running position of the video tape may be erroneously determined as the leading or trailing end of the video tape, even though it corresponds to the intermediate portion of the video tape. In this case, the running of the video tape is erroneously stopped. Even when the video tape reaches its leading or trailing end, it may run continuously. In this case, the video tape may be damaged.